Wireless communication systems commonly include information-carrying modulated carrier signals that are wirelessly transmitted from a transmission source (for example, a base transceiver station) to one or more receivers (for example, subscriber units) within an area or region.
A form of wireless communication includes multiple transmit antennae and multiple receiver antennae. Multiple antennae communication systems can support communication diversity and spatial multiplexing.
A Wireless Channel
FIG. 1 shows modulated carrier signals traveling from a transmitter 110 to a receiver 120 following many different (multiple) transmission paths.
Multipath can include a composition of a primary signal plus duplicate or echoed images caused by reflections of signals off objects between the transmitter and receiver. The receiver may receive the primary signal sent by the transmitter, but also receives secondary signals that are reflected off objects located in the signal path. The reflected signals arrive at the receiver later than the primary signal. Due to this misalignment, the multipath signals can cause intersymbol interference or distortion of the received signal.
The actual received signal can include a combination of a primary and several reflected signals. Because the distance traveled by the original signal is shorter than the reflected signals, the signals are received at different times. The time difference between the first received and the last received signal is called the delay spread and can be as great as several micro-seconds.
The multiple paths traveled by the modulated carrier signal typically results in fading of the modulated carrier signal. Fading causes the modulated carrier signal to attenuate in amplitude when multiple paths subtractively combine.
Spatial Multiplexing
Spatial multiplexing is a transmission technology that exploits multiple antennae at both the base transceiver station and at the subscriber units to increase the bit rate in a wireless radio link with no additional power or bandwidth consumption. Under certain conditions, spatial multiplexing offers a linear increase in spectrum efficiency with the number of antennae. For example, if three antennae are used at the transmitter (base transceiver station) and the receiver (subscriber unit), the stream of possibly coded information symbols is split into three independent substreams. These substreams occupy the same channel of a multiple access protocol. Possible same channel multiple access protocols include a same time slot in a time-division multiple access protocol, a same frequency slot in frequency-division multiple access protocol, a same code sequence in code-division multiple access protocol or a same spatial target location in space-division multiple access protocol. The substreams are applied separately to the transmit antennae and transmitted through a radio channel. Due to the presence of various scattering objects in the environment, each signal experiences multipath propagation.
The composite signals resulting from the transmission are finally captured by an array of receiving antennae with random phase and amplitudes. At the receiver array, a spatial signature of each of the received signals is estimated. Based on the spatial signatures, a signal processing technique is applied to separate the signals, recovering the original substreams.
FIG. 2 shows three transmitter antenna arrays 210, 220, 230 that transmit data symbols to a receiver antenna array 240. Each transmitter antenna array and each receiver antenna array include spatially separate antennae. A receiver connected to the receiver antenna array 240 separates the received signals.
Multiple antenna systems employ spatial multiplexing to improve data rates. In such schemes, multiple transmit signals are sent over separate antennas to obtain a linear increase in data rates. Spatial multiplexing schemes require no channel knowledge at the transmitter, but suffer performance loss in poor transmission quality channels. Poor transmission quality channels include properties that null out or attenuate some elements of the transmit signals. As a result, the receiver receives a badly distorted copy of the transmit signal and suffer performance loss. There is a need for additional transmit preprocessing schemes that assume channel knowledge and mitigate performance loss in poor transmission quality channels.
Communication Diversity
Antenna diversity is a technique used in multiple antenna-based communication system to reduce the effects of multi-path fading. Antenna diversity can be obtained by providing a transmitter and/or a receiver with two or more antennae. Each transmit and receive antenna pair include a transmission channel. The transmission channels fade in a statistically independent manner. Therefore, when one transmission channel is fading due to the destructive effects of multi-path interference, another of the transmission channels is unlikely to be suffering from fading simultaneously. By virtue of the redundancy provided by these independent transmission channels, a receiver can often reduce the detrimental effects of fading.
Multiple antennae systems typically employ antenna diversity to mitigate fading and ensure a robust communication link. Antenna diversity can be of two types—transmit diversity and receive diversity. Receive diversity is well known and involves combining signals from multiple receive antennae to mitigate fading. Receive diversity schemes require channel knowledge at the receiver. A method for obtaining transmission channel knowledge includes the transmitter sends known training sequences through the wireless transmission channel. The receiver estimates the transmission channel by comparing the received sequence with the known transmit sequence.
Transmit diversity is a relatively recent technology and involves sending multiple copies of the transmit signal on different transmit antennae. These multiple copies are typically include linear or non-linear mappings of the original transmit signal. Space-time coding is a powerful transmit diversity scheme that can be used to improve system performance. Transmit diversity schemes require no channel knowledge at the transmitter, but suffer performance loss in poor transmission quality channels. Poor transmission quality channels include certain properties that null out or attenuate some elements of the transmit signals. As a result, the receiver receives a badly distorted copy of the transmit signal and suffer performance loss. It is desirable to have transmitter schemes that can operate in poor transmission quality channels. Typically, such schemes require some channel knowledge at the transmitter.
Preprocessing
In some applications, channel knowledge is available at the transmitter. Channel knowledge at the transmitter can be acquired by receiving channel information from the receiver, or by the transmitter directly estimating channel information. In frequency division duplex (FDD) systems, a feedback link can be used to send channel information from the receiver to the transmitter. That is, the receiver estimates the channel through channel training, and feeds the resulting channel information back to the transmitter. In time division duplex (TDD) systems, transmission and reception from both the transmitter and the receiver occurs at the same transmission frequency. That is, both the transmitter and the receiver are actually transceivers that transmit and receive information to and from each other at approximately the same frequency range. Therefore, the transmitter can estimate the channel through training of signals transmitted from the receiver. The estimated channel can be calculated at the transmitter, and used for preprocessing signals to be transmitted.
When channel knowledge is available at the transmitter, channel dependent preprocessing of transmit signals can be employed to improve system performance. The channel dependent preprocessing is typically accomplished by a precoder or a pre-processer that is located within the transmitter. The precoder linearly or non-linearly maps the transmit signals on to the multiple transmit antennae to improve system performance. The precoder can be used in conjunction with or in lieu of existing diversity and multiplexing schemes.
The prior art includes several linear precoder schemes that rely on perfect channel knowledge at the transmitter. However, in wireless transmission systems, perfect channel knowledge can hard to obtain due to a number of reasons. For example, in an FDD system, the feedback link might be slower than the coherence time. That is, the channel may change before the transmitter receives feedback from the receiver. In this situation, the prior art precoding schemes suffer performance loss and are not useful.
It is desirable to have a method and system that include preprocessing of transmit signals in multiple transmit antennae systems that does not require perfect transmission channel knowledge. The method and system should be operational in existing diversity and spatial multiplexing systems, and should be able to account for transmission channels having slowly varying transmission channel characteristics.